Most of the conventional electrophotographic photoreceptors use light-sensitive layers that contain, selenium, zinc oxide, cadmium sulfide and other inorganic photo-conductors as the primary component. But none of these photoreceptors have satisfactory heat resistance, moisture resistance or printing life. Further, great difficulties are met in manufacture and handling those using selenium and cadmium sulfide due to their toxicity.
Electrophotographic photoreceptors using light-sensitive layers containing organic photo-conductive compounds are also known, and they are getting an increasing amount of researchers' attention these days because they are fairly easy to manufacture, the production cost is low, they can be fabricated into a cylindrical drum or sheet form, they are easy to handle, and unlike the selenium photoreceptor that is so low in heat resistance that it crystallizes under elevated temperatures, the new photoreceptors have high heat resistance.
Among the organic photo-conductive compounds, poly-N-vinylcarbazole is best known, and an electrophotographic photoreceptor having a light-sensitive layer that contains as the main component a charge transfer complex formed from the poly-N-vinylcarbazole and a Lewis acid such as 2,4,7-trinitro-9-fluorenone is currently used in industry. But even this photoreceptor does not have satisfactory sensitivity or printing life.
A two-layer or dispersed type photoreceptor wherein the carrier generating function and carrier transporting function are fulfilled by two different materials is known. This function-separated photoreceptor permits great latitude in selecting the proper materials, is fairly easy to provide better electrophotographic characteristics including charge acceptance, sensitivity, residual potential and printing life, and a photoreceptor having the desired characteristics can be fabricated. Various carrier generating materials have been proposed, and a carrier generating layer formed of amorphous selenium as an inorganic material is well known, but this has one great problem; under elevated temperatures, it crystallizes and deteriorates.
Photo-conductive organic dyes and pigments that have particularly high carrier generating capacity have been proposed for use as carrier generating materials. Among them are the perylene pigments described in U.S. Pat. No. 3,871,882, as well as the azo compounds disclosed in Japanese Patent Applications (OPI) Nos. 37543/72, 4241/77, 95033/78, 79632/79 and 69147/80 (the symbol OPI as used herein means an unexamined published Japanese patent application). But electrophotographic photoreceptors wherein these compounds are combined with carrier transporting materials do not have satisfactory characteristics such as sensitivity and residual potential, and if they have fairly good characteristics, such characteristics will not remain constant over extended use, and as they are subjected to cyclic operation, their sensitivity is decreased or the residual potential is increased. Some of the bisazo compounds described in the above patents commercially used, but photoreceptors using them are still vulnerable to temperature variations with respect to sensitivity, dark decay and residual potential.
In the electrophotographic photoreceptor using an organic photoconductive compound, carrier transfer is generally considered to take place by the hopping process, wherein the carrier transfer is accelerated by heat. Furthermore, carriers that have been captured in a relatively shallow trap are released by heat to generate a thermally stimulated current, which leads to an increased dark current.
Most bisazo compounds are known to have various crystal forms, which may be transformed by heat. There is a close relation between the crystal form of a specific bisazo compound and its electrophotographic characteristics, and depending on the crystal form, some bisazo compounds cannot be used commercially as light-conductive materials in photoreceptors.
The electrophotographic characteristics such as sensitivity, dark decay and residual potential are temperature-dependent. On the other hand, the temperature in an electro-photocopier has a tendency to increase during continuous operation due to the heat from the light source and fixing device. This tendency is increased as the size of the copier is decreased, and in small compact models, the surface temperature of the photoreceptor may reach as high 60.degree. C. On the other hand, in winter the same temperature may drop to 10.degree. C. or below in an unheated room.
If the sensitivity of the photoreceptor changes greatly with such temperature variation, it is difficult to produce a good copy image over extended use unless a cooling fan or an exposure control device such as aperature is installed in the copying machine. Alternatively, a trial printing is necessary for producing optimum copy image and this makes the copying procedure complicated. Therefore, a commercial photoreceptor for electrophotography should desirably be usable over a wide temperature range with minimum variations in its characteristics.